1. Field of the Disclosure
This disclosure relates acoustic sensors having high acoustic attenuation backings, methods of making such sensors and using same in acoustic tools for downhole well logging and other applications.
2. Description of Prior Art
Typical downhole acoustic logging tools utilize one or more acoustic transmitters and a number of acoustic receivers. The frequency of a typical transmitter is in the ultrasonic range, generally 100 kHz or more. Such higher acoustic frequencies are preferred in order to achieve better resolution in the confined space of a borehole. The tool is generally enclosed by a fluid in the wellbore. The transmitters often utilize piezoelectric transmitters in a ceramic or metallic enclosure. Transmitters transmit acoustic signals into the earth formation surrounding the borehole and the various receivers sense acoustic signals reflected by the formation. Such received signals are processed to determine a property of interest of the formation and/or of the borehole wall.
The active part of a typical ultrasonic transducer has a low ultrasonic absorption that causes it to ring when the transducer is excited. Such transducers typically contain a piezoelectric active element. A backing material is usually attached to the transducer backside to scatter and/or absorb acoustic energy to reduce ringing, and to attenuate acoustic signals to reduce unwanted reflections and reverberations in the backing. Acoustic attenuation and impedance (product of density and sound speed) are two important properties of a backing. The acoustic impedance of the backing for piezoelectric ceramic transducer materials generally range from a high value of approximately 30 MRayl to a low value of 2-4 MRayl, depending on the transducer design. Attenuation is the loss of acoustic energy and is mainly due to scattering of acoustic energy and absorption losses. Typically, acoustic transducer backing materials are constructed of a solid matrix, usually an epoxy or another polymer filled with solid fillers such as tungsten powder to increase the density, and tungsten particles or micro balloons, etc., that act as acoustic scatterers, to increase the attenuation. As acoustic waves pass through the backing, any motion of the filler particles relative to the matrix aids to inelastically attenuate the acoustic waves. In the case of a filler that is dense and has higher acoustic impedance than the matrix, the acoustic impedance of the backing material increases with the amount of the added filler. The composition of the backing is chosen to match the acoustic impedance of the backing to that of the transducer.
A porous media submerged in a fluid has been utilized to attenuate acoustic waves traveling in that fluid and to act as a blocking baffle to prevent acoustic waves from propagating in the fluid beyond the baffle. Such a device however has not been used as a direct acoustic transducer backing to absorb acoustic waves generated by a solid material. The disclosure herein provides a liquid-filled porous medium backing and transducers and acoustic tool using the same, which transducers are suitable for high temperature and wellbore applications.